Lateral PN arrayed digital X-ray image sensor

ABSTRACT

An X-ray imaging array is described together with a method for its manufacture. The array is defined by a set of PN junctions in a silicon wafer that extend all the way through between the two surfaces of the wafer. The PN junctions are formed using neutron transmutation doping that is applied to P-type silicon through a mask, resulting in an array of N-type regions (that act as pixels) in a sea of P-type material. Through suitable placement of the biassing electrodes, a space charge region is formed that is narrower at the top surface, where X-rays enter the device, and wider at the lower surface. This ensures that most of the secondary electrons, generated by the X-ray as it passes through the wafer, get collected at the lower surface where they are passed to a charge readout circuit.

FIELD OF THE INVENTION

The invention relates to the general field of X-ray detection by solidstate devices with particular reference to the formation of real timeX-ray images.

BACKGROUND OF THE INVENTION

Ever since the emergence of high tech. solid state technology, attemptshave been made to develop high resolution, low dosage X-ray imagingdevices. Ideally, such a device would allow a high resolution image tobe viewed in real time at a location well removed from the X-ray sourcewhile at the same time subjecting the object (or person) under scrutinyto a relatively low dose of X-radiation.

In FIG. 1 we illustrate what happens when a quantum of X-radiation 2enters a solid material 1 such as a crystal of silicon or germanium. TheX-ray will gradually dissipate over a distance proportional to itsenergy. The energy dissipated per unit length is referred to as thestopping power of the material, being 1.6 keV/micron for silicon and 3.2keV/micron for germanium. Thus the range of 100 keV X-rays is about 60microns in silicon and about 30 microns in germanium.

Two mechanisms are involved in dissipating the X-ray. First is theCompton effect which is a momentum exchange between the quantum and afree electron in the solid resulting in an increase in wavelength forthe former and an increase in energy for the latter. Second is thephotoelectric effect which is the ejection of an inner shell electronconverting it, at least temporarily, to a free electron. The net effectof the two mechanisms combined is to temporarily increase theconductivity of the irradiated material in a region immediatelysurrounding the trajectory of the X-ray. The higher the stopping powerof the detector material the smaller the volume of this region.

This effect of the X-radiation is the basis for a typical X-ray detectorof the prior art such as the one shown in schematic form in FIG. 2.Block 21 of silicon or germanium is divided into three parts. In thecenter is the detecting region 22 which is of intrinsic material(typically formed through lithium doping), while the two ends thatconnect to leads 23 and 24 are of P and N type material. In the absenceof X-radiation 2 the resistance between 23 and 24 is very high (so thereis negligible leakage current) both because of the intrinsic resistivityof 22 and because of the opposite conductivity types at the ends. Onceradiation is applied to 21, large numbers of electrons and holes areformed and the resistance between 23 and 24 drops so that a substantialleakage current can be measured.

The main disadvantage of this type of detector is that a relativelylarge(volume of semiconductor material, of the order of 10 cc., isrequired for adequate sensitivity. If smaller volumes are used, onlysome of the electrons formed in the wake of the X-ray will be availablefor collection and sensitivity is reduced. Thus detectors of this typeare unsuitable for use in imagers as they provide poor resolution.

Because of its complete compatibilty with integrated circuits, siliconwould be the preferred choice for an X-ray imager but, as discussedabove, its low stopping power prevents good pixellation, at least withthe PiN structures known to the prior art. Germanium has a largerstopping power for X-rays but it is still relatively small and, also,the intrinsic resistivity of germanium at room temperature is quite low(about 50 ohm-cm.). An example of a material that has large stoppingpower is cadmium zinc telluride (CZT) but this material is not availablein crystalline form. This results in low carrier mobility and shortminority carrier lifetime, leading to poor signal/noise ratio and slowresponse time. Similar problems apply to mercury iodide which suffersfrom the additional serious drawback of being extremely soft.

Because of the difficulties discussed above, the number of X-ray imagerdesigns described in the prior art is quite small. References that wehave found to be of interest include Antonuk et al. (U.S. Pat. No.5,079,426 January 1992) who describe an array of photoconductors made ofhydrogenated amorphous silicon. Snoeys et al. (U.S. Pat. No. 5,237,197August 1993) describe an array of PIN diodes which are biassed tocollect charge generated by ionizing radiation. The junctions areparallel to the wafer surface so occupy a fair amount of space. Lee(U.S. Pat. No. 5,652,430 July 1997) describes a structure that also usesa photocondcutor as the transducer. A charge accumulating capacitor isincluded with each detecting element for periodic discharge into adisplay.

SUMMARY OF THE INVENTION

It has been an object of the present invention to provide a device forimaging an X-ray beam.

A further object has been that said device have high resolution, highsensitivity, low cost, and operate in real time.

A still further object has been that the device be compatible withintegrated circuits and their packages.

Yet another object has been to provide a method for manufacturing thedevice.

These objects have been achieved by forming PN junctions in a siliconwafer, said junctions extending all the way through between the twosurfaces of the wafer. The PN junctions are formed using neutrontransmutation doping that is applied to P-type silicon through a mask,resulting in an array of N-type regions (that act as pixels) in a sea ofP-type material. Through suitable placement of the biassing electrodes,a space charge region is formed that is narrower at the top surface,where X-rays enter the device, and wider at the lower surface. Thisensures that most of the secondary electrons, generated by the X-ray asit passes through the wafer, get collected at the lower surface wherethey are passed to a charge readout circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the progress of an X-ray quantum through a block ofsemiconducting material.

FIG. 2 illustrates the PiN cell used in the prior art for the detectionof X-rays.

FIG. 3 is a schematic cross-section of an X-ray imager based on thepresent invention, also illustrating how such a device is manufactured.

FIG. 4 shows the basic operating principle of the device of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 3 illustrates both the main features of the device disclosed in thepresent invention as well as the process that is used to form it. P-typesilicon wafer 31 (between about 0.2 and 1.5 mm. thick, between about 10and 30 cm. in diameter, and having a resistivity between about 40 and150 ohm-cm.) has been subjected to bombardment by thermal (slow)neutrons (symbolized by arrows 32) through mask 33. The latter was madefrom a material with good neutron attenuation characteristics such asgadolinium or cadmium or their alloys and is between about 2 and 10microns thick.

As a consequence of neutron transmutation doping (NTD), those portionsof wafer 31 not protected by the mask were converted to N-type silicon(with resistivity between about 20 and 100 ohm-cm.) which extended allthe way through the wafer from surface to surface.

The NTD process is based on the fact that, although silicon has anatomic number of 14 and an atomic weight of 28, naturally occurringsilicon is not entirely made up of the Si²⁸ isotope. It turns out thatSi²⁹ is present at a concentration of about 4.7 atomic % and Si³⁰ ispresent at a concentration of about 3.1 atomic %. Additionally, it turnsout that Si³⁰, when bombarded by thermal neutrons, is transmuted tophosphorus P³¹ (atomic number 15). Since the desired level of phosphorusdoping is well below the 3.1 at. % of the already present Si³⁰, it isapparent that a limited amount of neutron bombardment of naturallyoccurring silicon, will result in the introduction of phosphorus dopantinto the silicon. Such phosphorus dopant will be uniformly distributedand will also be in substitutional position in the lattice where it canact as a donor.

Details of the NTD process used in this instance are as follows: aneutron fluence of between about 1×10¹⁸ and 5×10¹⁸ neutrons/cm.² whichcan preferably be accomplished by a flux of between about 10¹³ and 10¹⁴neutrons/cm.² sec. The period of time over which NTD was carried out wasbetween about 3 hours and 5 days.

The areas converted to N-type by the NTD were defined by the openings inmask 33. These openings were generally square but; other shapes, such ascircles, could also have been used. The linear dimension (square side,circle diameter, etc.) of the openings was between about 40 and 100microns and their area was between about 1,600 and 10,000 sq. microns.The distance between openings was between about 10 and 30 microns,corresponding to an array density of between about 6,000 and 40,000pixels/sq. cm. made up of N-type areas surrounded by P-type material.

Referring now to FIG. 4, we illustrate the operation of a single pixelof the display. We note first that X-rays (symbolized by arrow 41) enterfrom the top (or near) side. Each pixel has four sets of contacts towhich different voltages will be applied. The contacts 51 connect to theP region immediately adjacent to the pixel and are connected to voltageV1. Contacts 52 connect to N+ region 46 which was separately formedusing either ion implantation or chemical diffusion. Contacts 52 areconnected to voltage V2 and are located close to PN junction 38.

Typical values for V1 and V2 are -200 and -100 volts, respectively.

Contacts 53 are on the far side of P region 35 but are located close tothe PN junction 38 (for reasons that will be explained below). They areconnected to voltage V3. Contact 54 covers most of the far side of thepixel and is connected to voltage V4.

Typical voltages for V3 and V4 are +50 and +100 volts, respectively.

Output line 59 runs from contact 54 to a suitable charge readout circuitsuch as, for example, existing circuitry from a charge coupled device(CCD). The pixel is activated when all four voltages V1-V4 are in place.As a result of the voltage differences between V1 and V4, the PNjunctions 38 are reverse biassed and a depletion or space charge region(SCR) 61 is established. The application of V3 at contact 53 causes theSCR to be extended to include volume 61a while the application of V2 atcontacts 52 pulls the SCR in towards the PN junction 38.

Thus, through the above described placement of the various biassingcontacts, together with the voltages applied to them, a SCR is formedthat is narrower at the near end, where X-rays enter the display, andwider at the far end. An X-ray quantum starting its journey at thecenter of the pixel encounters a high density of free electrons therebyincreasing the probability of forming Compton effect electrons. As theseprogress, together with the original quantum (now reduced in energy),they enter the SCR inside which all but the highest energy (>300 V)electrons are channeled towards electrode 59. Since the electron densityis highest near the far side of the pixel, the widening there of the SCRincreases the probability that they will lose sufficient energy to beretained within the SCR before they reach its edge. In a conventionalX-ray detector of the prior art this had to be achieved by means of thelarge intrinsic region.

This completes the description of the device, and the process formanufacturing it, except to note that the device, as described, is wellsuited for incorporation within a standard integrated circuit package.For example, contacts 51 and 52 could take the form of solder bumps sothat the chip or wafer could be mounted using flip chip technology whilecontacts 53 and 54 could be accessed using wire bonding technology.

The design of the present invention has a collection efficiency (for 100keV X-ray induced electrons) of about 2% for a 500 micron thickdetector. This corresponds to the collection of about 100 electrons perincident photon or 10⁶ electrons by a 40×40 micron pixel irradiated with100 keV X-rays at an intensity of 10⁴ photons per pixel.

While the invention has been particularly shown and described withreference to the preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in for if and details maybe made without departing from the spirit and scope of the invention.

What is claimed is:
 1. A process for forming an X-ray imaging device,comprising:providing a wafer of P-type silicon having an upper and alower surface; through a mask having an array of openings, bombardingthe wafer with properly aligned thermal neutrons for a period of time,thereby converting to N-type all silicon located below said openings andthereby forming an array of P-type regions surrounded by N-typematerial; wholly within each P-type region, forming an N+ region thatextends downwards from said upper surface; on the upper surface, whollywith each of said regions of N+ silicon, forming first negatively biasedcontacts; on the upper surface, wholly within each of the P-type regionsand outside the N+ regions, forming second negatively biased contactsmore positive than the first negatively biased contacts on the lowersurface, centrally located within each P-type region and underlying eachregion of N+ silicon, forming first positively biased contacts; and onthe lower surface, underlying the second negatively biased contacts,forming second positively biased contacts more negative than the firstpositively biased contacts.
 2. The process of claim 1 wherein the waferhas a thickness between about 0.2 and 1.5 mm.
 3. The process of claim 1wherein each of said openings has an area between about 1,600 and 10,000sq. microns.
 4. The process of claim 1 wherein the openings are betweenabout 10 and 30 microns apart.
 5. The process of claim 1 wherein themask is selected from the group consisting of gadolinium, cadmium, andalloys of gadolinium and cadmium.
 6. The process of claim 1 wherein thestep of bombarding with thermal neurons further comprises using aneutron fluence of between about 1×10¹⁸ and 5×10¹⁸ neutrons/cm.².
 7. Theprocess of claim 1 wherein the step of bombarding with thermal neuronsfurther comprises providing a neutron flux of between about 10¹³ and10¹⁴ neutrons/cm.² sec.
 8. The process of claim 1 wherein said period oftime is between about 3 hours and 5 days.
 9. The process of claim 1wherein the P-type regions have a resistivity between about 40 and 150ohm-cm.